ANTIMICROBIAL: Erythromycin

CLASS: Macrolide

SOME OTHER ANTIMICROBIALS IN THE SAME CLASS: Clarithromycin, azithromycin, telithromycin, roxithromycin & others.  Note not all “mycins” are macrolides, with erythromycin (macrolide) and vancomycin (glycopeptide) being the ones most commonly mislabelled.

ORIGINS: In 1949, a research team from Eli Lilly isolated erythromycin from a soil bacterium, Streptomyces erythreus (now renamed Saccharopolyspora erythraea). The antibiotic was launched commercially in 1952.

MECHANISM OF ACTION: Erythromycin is a bacteriostatic antibiotic. It binds to the 50s subunit of the bacterial- 70s rRNA complex, thus preventing transfer of tRNA.

ROUTE OF ADMINISTRATION: Oral, intra-venous.

MAIN CLINICAL USES: Soft tissue & pharyngeal infections, particularly when the patient has a history of allergy to penicillin. Cover against “atypical” pathogens in community acquired pneumonia, such as Mycoplasma, Legionella and Chlamydia. Treatment of pertussis.

MAIN SIDE EFFECTS OF NOTE: Because erythromycin has pro-kinetic activity, gastro-intestinal upset is very common and can hinder the practical usefulness of the drug. The most serious side effect is it’s effect on the conduction system of the heart, with potential to cause rare but life-threatening arrythmias by prolonging the “QT interval”. Has also been linked to pyloric stenosis in very young infants.

RESISTANCE: In staphylococci and streptococci, resistance to macrolides is conferred either by methylases encoded by genes such as erm, that modify the ribosomal target of macrolides, or by pumps that efflux these antibiotics (eg mef gene). Cross resistance may occur between macrolides, lincosamides and streptogrammin B compounds. See this link.

OTHER POINTS OF NOTE: May have an adverse effect on the effectiveness of the combined oral contraceptive pill. Erythromycin also inhibits cytochrome P450, so may potentiate effect of other drugs metablised through this system, such as warfarin.